Optimum Vision and Eye Protection
Of the many tasks required of pilots and controllers, nearly all use vision as a critical source of information input and assessment of outcome. Indeed, visual input is the most critical source for pilot/controller judgment and decision making. What can they do to protect and enhance their vision? This article offers tips in several areas.
Many hazards to vision exist. All can jeopardize a pilot or controller’s career. Some hazards can rob a pilot/controller of vision in one eye very suddenly, while others may affect both eyes very gradually. This section is divided into vision protection suggestions for physical hazards, radiation hazards, health hazards and nutritional hazards. Visual scanning techniques for day and night flying are reviewed. Sunglasses for pilots have very specific requirements. Eye strain and fatigue avoidance will improve vision. FAA policy concerning waivers for vision problems are outlined.
Many people lose their vision to preventable eye injuries each year. These injuries occur at the work place, during participation in sports, in motor vehicle accidents and in casual activities around the house. Nearly 90% are preventable. Traumatic injuries tend to occur suddenly with potential total loss of vision in one or both eyes.
Active sports participants should use eye protection if available. Polycarbonate “sports” lenses are shatter resistant and can stop a .22 caliber bullet. Vision protection is absolutely necessary in indoor racquet sports, such as squash and racquetball. High contact sports such as hockey and lacrosse also put the eye at risk for serious injury. Many football players are now using face shields to prevent career ending injuries. Hunters, trap shooters and target marksmen often wear protective lenses that enhance contrast and avoid eye injuries. Baseball players may wear polycarbonate lenses to protect their eyes, even if they do not need glasses for better vision.
Eye injuries on the worksite may affect even full-time pilots and controllers. Certainly those who do their own maintenance have experienced sprays of hydraulic fluid, metallic chips or (hopefully not!) flakes of rust near or in their eyes. Rust particles can permanently stain the cornea (clear part of the front of the eyes) while a metal splinter coming off a hammer may penetrate the globe of the eye. Simple and inexpensive soft plastic goggles can prevent all of these injuries. For those working with welding equipment, additional protection is required.
Many permanent eye injuries occur in the home or with recreation. Wood chips and splinters thrown from power saws, rust and petroleum products from working underneath a car and playful pets with claws all cause serious injuries. Trimming bushes that reach to eye height often leads to individuals focusing on one branch and walking into another. For the free spirits who ride motorcycles without helmets, goggles or glasses, a bug in the windstream or a rock thrown from a tire can be disastrous. Having goggles or wrap around glasses can prevent each of these injuries.
There are many forms of electromagnetic radiation. The primary forms of concern to pilots and controllers include both ionizing and non-ionizing. Non-ionizing radiation includes visible, near-visible (ultraviolet and infrared) and radiofrequency. In general, aircraft cockpit windscreens, fuselages, and tower cab windows block any significant ionizing radiation that may affect the eye. Visible light and ultraviolet (UV) light require sunglasses for comfort and improved visual acuity depending on the intensity, but UV light does not present hazards when in an aircraft.
Sunglasses worn on bright days also improve night vision if a pilot is flying from daylight into darkness. Outside an aircraft in direct or reflected sunlight, protection from UV light is desirable (see sunglasses for pilots below). UV-B light less than 315 nanometers in length, the same type that causes sunburns and skin cancer, can induce cataracts, but is blocked by plain glass and polycarbonate. It will penetrate acrylics and soft plastics found in cheap sunglasses. It also contributes to macular degeneration.
Infrared (IR) radiation is perceived primarily as heat. It is not a hazard to vision. Microwave radiation from radar’s may accelerate cataract formation if an individual repeatedly stands in front of a radar that is on. The microwave radiation is converted into heat energy when it is absorbed by the lens. The heat causes the proteins of the lens to “clump” and form a focus for a growing cataract which will obscure vision.
Weather radar’s are generally lower power and will not cause as significant visual problems as will military search radar’s. Incidental exposures, such as performing a pre-flight on an airliner with the weather radar powered on a single time, is unlikely to cause any damage. Although microwave ovens are supposed to be shielded, it would be wise not to stare closely at food cooking in a microwave oven to avoid possible IR damage to the lens of the eye.
Eye Strain and Vision Fatigue
As we move into the computer age, eye strain at work and at home is a more frequent phenomenon. The eye focuses by using muscles to change the shape of the lens, thereby adjusting for different working distances. As we age, the lens becomes less pliable and focusing at near becomes more difficult. This condition is called presbyopia.
When we stare at a computer monitor for extended periods of time, we are asking the ciliary muscles of the eye to constantly contract a fixed amount to keep the screen in focus. Just as a biceps muscle fatigues if someone holds an object in a constant position (remember your student pilot days holding the yoke for a straight and level attitude without proper trim), the eye muscles fatigue if focusing at a fixed distance. As a result, the eyes may get more blood flow (turn red), get dried from not blinking (increased tears to compensate) and not be able to rapidly change focus.
The key to prevention of eye strain and fatigue is to take frequent breaks and focus the eyes at a distant object. This allows the ciliary muscles to relax. Computers do not emit enough radiation to be a hazard to vision.
Likewise, reading for long periods of time or reading in the dark will cause eye fatigue. Neither will cause any permanent damage, but both will lead to temporary problems focusing, redness, tearing and headaches. Overly bright conditions or large amounts of reflected light cause the pupils to constrict, also using eye muscles to constantly adjust the light reaching the retina. Although these are different muscles than those changing the shape of the lens, the same symptoms of fatigue occur. The key to preventing this type of fatigue is to read in a comfortable level of light and take frequent breaks.
Sunglasses for Pilots
There are many types of sunglasses available, but no single type is ideal for every pilot. Needs change based on age, light sensitivity, ambient lighting conditions and type of flying. Some sunglasses are not right for any pilot at any time. Valid reasons for wearing sunglasses in the aviation environment include improved night vision adaptation, enhanced contrast in the visual field, reduced glare, decreased UV exposure and avoidance of eye fatigue. Though style and appearance may be a consideration, the safety conscious pilot should focus on the proper selection of lens features rather than frame styles with cheap lenses.
Visual acuity varies with the light available and the sensitivity of an individual to various degrees of brightness. The pupil controls the amount of light reaching the retina. Older individual’s eyes do not transmit as much light through the eye as younger people do. Therefore, many older individuals need more light for optimum acuity. They may want to use sunglasses that transmit more light. On high glare days, such as over snow or sand, the pupils contract to protect the eye from the glare. Sunglasses will reduce glare and allow the pupil to let more light on to the retina, thus enhancing vision.
Sunglasses for Pilots – Glare
Glare can also be caused by indirect blue wavelength light and UV light. UV light increases by 4% for every 1,000 feet of altitude and contributes to the blue color of the sky. Some researchers feel this can cause haziness on the retina decreasing visual acuity even when indirectly viewed. Fortunately, most windscreens eliminate much of this wavelength. Near sunrise and sunset, the atmosphere filters out this wavelength giving the sky its characteristic red-orange color. Yellow lenses, often called “blue blockers” will block out this wavelength also and may improve vision on a hazy day. They may present decreased perception of some cockpit displays, however.
Sunglasses for Pilots – Tinted lenses
Tinted lenses distort colors to some extent. The yellow shaded “blue blockers” will alter color perception if tinted enough to block out 30% of the light. Thus, these lenses should be used only by aviators on bright, hazy days and avoided in low light situations. Green and gray lenses have the least distorting effect on color vision. Brown distorts colors slightly more, but can block some of the blue light blurring in haze.
Sunglasses for Pilots – Darkness
Darkness or degree of light reduction is calculated in percentage and listed by numbers. A #1 lens blocks only 20% of the incoming light and has little value for aviators. The exception may be the #1 Yellow lenses for hazy or smog filled days. The #2 lens blocks 70% of light and are useful for most aviation situations. It provides a balance of glare protection, luminescence reduction and UV protection without significantly reducing visual acuity. The light reduction of #3 (85%) may be useful for those pilots particularly sensitive to bright light while others may find the #3 lens reduces visual acuity. The #4 lens blocks out 95% of incoming light and significantly reduces visual acuity because the macula, where sharpest vision is found on the retina, requires light to activate the cones of the retina. Aviators wearing these lenses in flight will not meet FAA minimum distant visual acuity standards.
Sunglasses for Pilots – Mirrored glasses
Mirrored glasses use metal particles to reflect images. They scratch easily and can cause distortion or blind spots. While popular for Hollywood movie pilots, professional pilots should leave them to actors, policemen and other imitators.
Sunglasses for Pilots – Photochromatic lenses
Photochromatic lenses increase darkness when exposed to UV light. Because aircraft windscreens block most UV light, the lenses will not darken substantially inside an airplane or car. The military prohibits its pilots from using these sunglasses for good reason. Pilots flying open cockpit airplanes are the only ones who may benefit from this feature.
Sunglasses for Pilots – Gradient lenses
Gradient lenses usually have a darker tint on the upper portion of the lens and a gradually lightening color near the bottom. This may be useful when trying to view instruments on a very bright day. The lighter tinting below allows more light from the relatively dark instrument panel to reach the retina and improve visual acuity while blocking out the glare from the outside view.
Sunglasses for Pilots – UV protection
UV protection is desirable for lenses worn outdoors, although it is not as important for glasses worn inside the aircraft since this wavelength is already blocked by the windscreen. Glass and polycarbonate block nearly all of the UV-B light. Soft plastic lenses may block visible light but not block any UV wavelengths. The hazard for cataract formation in the individual using soft plastic lenses is increased because the pupil dilates in response to decreased visible light. The dilated pupil allows more UV light to enter and penetrate the lens increasing the risk of cataracts.
Sunglasses for Pilots – Scratch resistant coating
Scratch resistant coating may increase the life of polycarbonate lenses and plastic lenses. Ironically, polycarbonate will withstand direct hammer strikes without breaking, but scratches relatively easily. Glass will shatter, but is more resistant to scratching. Polycarbonate lenses are thinner and lighter than glass lenses.
Sunglasses for Pilots – Polarized lenses
Polarized lenses should not be worn by pilots in the cockpit. Glare from flat surfaces is blocked by polarized lenses which are oriented in parallel lines like closely spaced prison bars. Light parallel to the lines is transmitted while non-parallel light (glare) is blocked. Unfortunately, if the windscreen is polarized and the lenses are not precisely oriented the same as the windscreen, all light may be blocked. Changing bank angle and head position could create blind spots. For those who are boating and need glare protection from light reflected off the water, polarized lenses are excellent choices.
Sunglasses for Pilots – What we recommend
What we recommend: On bright days, consider using a neutral tint (green or gray) glass or polycarbonate lens that blocks 70-90% of the incoming light, possibly with a gradient that lightens on the lower portion of the lens. On a hazy or smog filled day, consider a yellow or brown lens that blocks 20% of the light, but avoid using it if color perception (IFR flight), as opposed to visual acquisition (VFR flight), is important. The military has found that some fighter pilots prefer the “high contrast” yellow visors for their helmets to enhance target acquisition, while others prefer not to wear yellow visors. At dusk or in lighting that is comfortable without sunglasses, remove them to increase visual acuity. Don’t use polarized or photochoromatic lenses in the cockpit. Don’t waste your money on soft plastic lenses or mirrored lenses. Scratch resistant coating may increase the life of polycarbonate lenses.
Nutrition for Optimum Vision
Many people think of the role of nutrition in lowering the risk of heart disease, reaching ideal body weight, controlling diabetes and other medical conditions. Few people consider nutrition in optimizing their vision, a critical function for pilots and controllers. Current research supports the role of several nutrients in visual health.
Next to errors in visual refraction, cataracts are the most important cause of degraded vision in pilots and many controllers. This is particularly true after age 50 years. Over 1.5 million cataract surgeries are performed in the U.S. each year, costing $6 billion. It is the largest single item on the Medicare budget. Over 40,000 Americans are legally blind from cataracts. The hazards of radiation in causing cataracts are discussed above. Cataracts are a surgically treatable cause of vision loss. Several nutrients have been demonstrated to lower the risk of cataracts.
Vitamin C is a powerful antioxidant that may protect the lens of the eye from the free radical formation caused by radiation and aging. The lens concentrates vitamin C sixty times greater than in the blood. Because the lens does not have any direct blood supply, dosages of 1,000 mg per day may be necessary to raise vitamin C levels in the lens. This dose significantly reduced the requirement for cataract surgery over 11 years in 450 patients with cataracts compared to those not taking vitamin C in an older study in Archives of Internal Medicine.
Vitamin E may also lower the risk of cataracts. The May 1998 issue of Ophthalmology studied the growth of cataracts in 754 individuals. Those using a multivitamin supplement containing vitamin E reduced the rate of cataract formation. In those taking a regular vitamin E supplement and having higher blood levels of vitamin E, cataract formation was reduced by half. Doses of 400-800 IU per day are recommended. In individuals taking either 300 mg of vitamin C or more than 400 IU of vitamin E had less than half the amount of cataracts as those who did not use supplements.
Selenium is an essential cofactor in the breaking down of hydrogen peroxide, a free radical, in the liquid filled chamber in the front of the eye called the aqueous humor. The enzyme that does this, glutathione peroxidase, depends on selenium. One study showed reduced levels of selenium in the blood and aqueous humor of people with cataracts, but no difference in the selenium levels in the lens itself. N-acetyl-cysteine is a precursor to glutathione and helps maintain vitamin C levels in the eye. Zinc may also play a role in reducing the risk of ARMD and studies are ongoing, but definitive evidence is lacking.
A second significant cause of vision deterioration is Age Related Macular Degeneration (ARMD). It is the leading cause of blindness over age 65. ARMD is generally not treatable and results in permanent blindness. Over 11 million Americans have ARMD, including up to 20% of those aged 65 and greater. The macula, as described below, contains the rods of the retina responsible for our sharpest vision. People affected with ARMD lose the ability to drive, read and even recognize faces. Obviously, a pilot and controller’s medical certificate is jeopardized even in early ARMD.
When your physician looks in your eye, the macula appears yellow because of the high concentration of carotinoids, especially lutein and zeaxanthin. Many of these carotenoids are building blocks of vitamin A and beta-carotene. As well as improving night vision, these nutrients may exert an antioxidant effect that protects against long term UV exposure contributing to ARMD and stabilize capillary (blood vessel) membranes in the retina.
The National Eye Institute’s Eye Disease Case Control Study showed individuals with higher levels of carotenoids had lower risks of ARMD. Lutein, zeaxanthin, alpha and beta-carotene, lycopene and cryptoxanthine all contributed to carotenoid levels. Vitamins C and E and selenium did not affect the risk of ARMD.
Lutein is a well studied carotenoid that does not form vitamin A. Humans deposit lutein and zeaxanthin in the macula. They protect the macula from oxidative damage caused by free radicals triggered by UV and visible blue light. Lutein is 10 times more effective as an antioxidant in the eye as is vitamin E. Studies in the Archives of Ophthalmology show that the higher the concentration of lutein, the lower the risk of ARMD. The Journal of the American Medical Association reported in 11/94 that 6 mg of lutein per day lowered the risk of ARMD by 43%. Several other studies support the role of lutein in reducing the risk of ARMD.
Bilberry is a plant related to the blueberry and cranberry. It contains a high concentration of flavinoids and anthrocyanosides. They exert an antioxidant effect and improve ocular circulation. Bilberry also improves night vision by stimulating rhodopsin production, an essential pigment in the rods used in low light condition vision. Anecdotally, there is a story of one RAF unit during the Battle of Britain had much higher kill rates at night than other nearby units. The flight surgeon researched differences in the units and found the only notable difference was that the higher scoring squadron regularly used bilberry jam at tea time since bilberries were found in the wild near the base. Although the explanation was not apparent at the time, current research supports this observation.
Glaucoma – Regular Check-ups Save Vision
Age Related Macular Degeneration affects the central vision primarily and is readily apparent to those afflicted with it. Glaucoma is a much more common condition that is not readily apparent to its victims. Over 2 million Americans have glaucoma, but only half have been diagnosed. Nearly 80,000 are blind from this entirely preventable condition.
The Intraocular Pressure (IOP) of the fluid in the eye is usually measured by a machine that gives a puff of air against the cornea. This pressure is compared to “normal” pressures, similar to the way an individual’s blood pressure is compared to normal levels. Borderline levels, called ocular hypertension, are observed and higher levels are treated with medication. Glaucoma can also be treated with laser surgery.
In glaucoma, chronic elevations of IOP are thought to cause damage to the optic nerve at the back of the eye. There is no pain associated with most forms of glaucoma and no early symptoms. If left untreated, a person begins to lose peripheral vision. This vision loss is permanent. Because the peripheral visual fields are lost initially, many victims do not notice this visual loss until the vision is restricted to a view similar to looking through a tube or round window. Obviously, a pilot’s ability to clear for other aircraft is restricted by this condition.
The FAA medical examination does not include a check of IOPs. Visual fields are noted on the exam, but even if performed correctly, only detects late changes. The FAA will waiver pilots to fly and controllers to control on most anti-glaucoma medications or after surgery for glaucoma. Prior to returning to safety sensitive duty with a diagnosis of or treatment for glaucoma, a pilot/controller must have an FAA Form 8500-14 completed by an eye care specialist with complete visual field testing done.
Diabetes is the leading cause of blindness in working age Americans. Diabetes leads to blindness by causing a condition known as diabetic retinopathy. Blood vessels in the back of the eye grow rapidly and leak blood into the back chamber of the eye called the vitreous. These blood leaks block vision. The retina can also detach which can lead to sudden blindness. Poor blood sugar control can lead to leaking of serum into the retina called exudates. Cataract formation is also increased in diabetes. Diabetes and its implications for pilots and controllers is discussed in the AMAS article on Diabetes on our website. This is a very serious medical condition that requires careful evaluation, monitoring and treatment. Vision changes are late finding with diabetes and indicate that other organ systems are probably affected also.
Visual Scanning Technique
The “see and avoid” rules for VFR flying (and IFR in some cases) are essential for safe aviation. Effective visual scanning techniques depend on many factors. These factors include good vision, a current lens prescription if needed, ocular health and nutrition, appropriate sunglasses and a very effective visual scanning technique. Visual scanning techniques in the daytime and at night vary significantly, if fact, they are nearly opposite from each other. While some of the other factors affecting visual clearing for other aircraft are dependent on natural visual health and available equipment, visual scanning techniques can be learned (and forgotten).
The portion of the eye that contributes to focusing images and sending those images to the brain is called the retina. The retina is found on the inner lining of the back portion of the eyeball. The retina contains rods and cones.
Rods are critical in low light and night time (scotopic) vision, but do not have very good visual acuity (focusing ability). Experts estimate that the best visual acuity using inputs from rods varies from 20/60 to 20/200 or worse.
Rods are located in the periphery of the retina and are about 100,000 times more sensitive to light than cones. Rods are also critical in detecting motion and provide peripheral vision. They do not detect colors, but perceive objects in shades of gray.
Cones are found in the central portion of the retina in a small area of about 10 degrees diameter called the macula. A condition known as Age Related Macular Degeneration severely degrades vision on this area. The central 1-2 degrees of the macula is called the fovea, which detects the finest details. The cones are responsible for vision in bright light conditions (photopic vision). The cones provide the best visual acuity and color vision. Outside this area, visual acuity drops off by 90% as the concentration of cones decreases and the concentration of rods increases. Because fovea contains no rods for night vision, it is also called the “night blind spot.”
Vision in twilight conditions is call mesopic vision and uses a combination of rods and cones. The eye also contains a “physiological blind spot” where the optic nerve enters the eyeball and the retina is devoid of any rods or cones. This area is about 15 degrees off the central visual axis and fovea. With different parts of the retina providing critical inputs in different lighting conditions, you can see that visual scanning techniques vary depending on the level of available light in the area scanned and the contrast between the target and its background.
Visual Scanning – Day Time
Because the best visual acuity comes from the central portion of the retina called the fovea, only a small cone of vision directly in front of the eye about 10 degrees in diameter actual focuses on objects and provides details with color vision. Recall that areas outside this central cone of best visual acuity detect motion in shades of gray. In day time, the best scanning technique involves focusing on segments of the sky in 1-3 second intervals. This allows you to detect small objects directly in the area you are focusing in, while letting your peripheral vision detect motion. After a brief search of one section of sky, shift your focus to another section about 15-20 degrees offset from the original area. Repeat the search in a stop-turn-stop method. If you detect motion in your peripheral vision, shift your focus to that area to pick out details and colors.
A sweeping technique across the horizon in daytime is not effective. Your eyes do not have time to focus on any one section of sky for object detection and the constant motion of the eye negates the ability of the peripheral vision to detect motion. Scan at, above and below the horizon. Be sure to scan laterally also. FAA mishap figures show that you are five times more likely to experience a midair collision with an aircraft moving in the same direction as your aircraft as you are to experience a head-on midair collision.
Visual Scanning – Night Time
Night time visual scanning is completely different from day time scanning. It is not intuitive and therefore, must be learned and practiced. Recall that there is no ability to focus directly on objects in low light conditions (night blind spot). You must rely on images arriving on the rods in the periphery of the retina. A pilot searching for objects at night should look off-center about 10-15 degrees from where it is expected. Once it is detected, looking directly at it will cause it to disappear. Instead, the Air Force teaches a “diamond scan” technique. This involves looking above, below and to each side of an object in a diamond shaped scanning pattern to keep it from bleaching out and disappearing.
Night vision is affected by many factors. Light is the most significant. Dark adaptation takes about 20-30 minutes. A single exposure to a bright light will cause night vision to deteriorate until the adaptation can occur again. If outside during the day or flying, sunglasses should be worn to allow full night adaptation to occur as lighting diminishes. Cockpit lighting should be dimmed to the minimum necessary to see instruments while allowing maximum night vision adaptation.
Red filters on lights will also prevent washout of night vision, but red writing and lines on maps, charts and operating handbooks may not be visible if viewed with red filtered light. Nutrition is critical as mentioned above. Vitamin A and beta-carotene are critical nutrients for optimum night vision. Smoking degrades night vision by about 20%. Vision is also very sensitive to oxygen levels. The retina uses large amounts of oxygen, particularly at night. Oxygen supplementation is recommended at cabin pressures above 5,000 feet MSL at night.
AMAS Aeromedical Assistance
For a more specific personal explanation to your questions or those concerning aeromedical certification, contact AMAS for a private consultation. For help in reporting treatment for and obtaining clearance from the FAA to fly and control with these conditions, refer to the AMAS Confidential Questionnaire. If you are an AMAS Corporate Member, these services are FREE to you.